1. Field of the Invention
The present invention relates to image forming apparatuses. More particularly, it relates to a technology for detecting failure which has occurred in an exposure section which performs light exposure by irradiating a light to a surface of a photosensitive member.
2. Description of the Related Art
Conventionally, in an image forming apparatus adopting an electrophotographic method, a light scanning device has been adopted which irradiates a light to a surface of a photosensitive drum by reflecting a laser light beam irradiated from a light source such as a laser diode with a light deflector such as a rotating multifaceted mirror to form an electrostatic latent image on the surface of the photosensitive drum. In such light scanning device, a BD sensor which receives a laser light beam at a predetermined position is provided, and a technology for setting a start timing (start position) of light beam scanning with use of an output signal (horizontal synchronizing signal) of the BD sensor has been disclosed in, for example, Japanese Patent Unexamined Publication No. 2003-200609.
In an image forming apparatus capable of performing color image forming, for the purpose of providing image forming units for respective colors, thus an exposure section and a laser diode are provided for each color. Further, for the purpose of speeding up the operation in an image forming apparatus, there has been also adopted a mechanism for simultaneously scanning a plurality of lines with use of a plurality of laser diodes provided in one exposure section. In the scanner unit provided with a plurality of laser diodes, there is a case where the BD sensor for horizontal synchronization is provided, for example, in a ratio of one for two (two colors) laser diodes, for the purpose of reducing the cost. In such case, a laser light which enters the BD sensor is either one of the two (two colors) laser diodes.
The laser diode may be deteriorated by electric stress due to a static electricity and a surge. The deteriorated laser diode weakens the light quantity of emitted light and cannot obtain the light quantity necessary for image forming, thereby causing a phenomenon which makes the density of a developed image be thin. However, as described above, in the case of the image forming apparatus adopting the configuration of performing horizontal synchronization of two (two colors) laser diodes with one BD sensor, failure in the laser diode which is so set as to receive the light at the BD sensor can be detected based on no reception of the light at the BD sensor, but failure in the laser diode which is so set as not to receive the light at the BD sensor cannot be detected in the aforementioned method. Therefore, there is likelihood that printing is continued even if there is failure in the laser diode which is so set as not to receive the light at the BD sensor.